I have already run 1/0 ser cable single phase wire for an apartment feeder to the sub panels. The inspector is telling me I cannot bunch them. I have run them down a hall and had to bunch them approx 18" to get into the panel room. He is saying I have to derate the wires. Is this true? If so it will cost much money.

When you say bunch do you mean bundle? If so then this only applies if it is over 24".
The references are NEC: 338.10(B)(4) , 334.80 & 310.15(B)(2)(a)

If this area is required to be fire stopped then the 24" rule does not apply.